1. Field of the Invention
This invention relates to an electrode for use in electrochemical processing having improved oxidation and corrosion resistance in comparison to prior art electrodes used for the same purposes, which can be readily produced by a process involving combustion synthesis to form a core body having an interconnected network of a ceramic or metal-ceramic composite in which is uniformly dispersed a filler material. Although not so limited the invention has particular utility in the provision of an anode and a cathode for the electrowinning of aluminum from its ore in the Hall-Herault process. The electrode of the present invention provides improved oxidation resistance at temperatures above 1000.degree. C. with retention of satisfactory electrical and thermal conductivity at such elevated temperatures.
2. Description of the Prior Art
U.S. patent application Ser. No. 07/648,165, filed Jan. 30, 1991, in the names of Jainagesh A. Sekhar and Sarit B. Bhaduri, and assigned to the assignee of the present application, discloses a composite electrode for electrochemical processing and a method for preparation thereof by combustion synthesis. Such an electrode comprises at least 20% by weight of a ceramic composite or a metal-ceramic composite in the form of a dimensionally stable interconnected network, at least about 15% by weight of a filler material providing desired electrochemical properties, the filler material being uniformly dispersed in the composite network, and up to about 35% by weight of a binder phase associated with the network and the filler material. The ceramic or metal-ceramic composite network is derived from a wide range of combustible mixtures which, when ignited, form the composite interconnected network or core. Filler materials are selected from a variety of nitrides, oxides, borides, carbides, silicides, oxyfluorides, phosphides, metals, and/or carbon. While the compositions and method of preparation of this application produce a dimensionally stable product, it has been found that electrodes made in accordance with the process are not stable above 1000.degree. C.
"Encyclopedia Of Materials Science", Vol. 2, Michael B. Bever, editor-in-chief, Pergamon Press, 1986, page 1413, summarizes the state of the art relating to electrode materials for electrochemical processing, including electrochemical research, electrolytic production of hydrogen, chlorine, chlorates, perchlorates, electrowinning of aluminum, and other electrochemical processes. At page 1413, a discussion of the electrometallurgy of aluminum points out that electrolysis of a cryolite-alumina (Na.sub.3 AlF.sub.6 +Al.sub.2 O.sub.3) melt is carried out using a carbon anode and an aluminum cathode to yield aluminum on the basis of the reaction: EQU 2Al.sub.2 O.sub.3 +3C.fwdarw.4Al+3CO.sub.2
Carbon dioxide is formed at the anode. The types of carbon anode presently used are described, and it is also pointed out that carbon is used as a cell lining in the reduction cell. Lining failure and anode consumption are recognized as being major disadvantages in the present process.
The use of combustion synthesis (CS), also referred to as self-propagating high-temperature synthesis (SHS), for a variety of applications is reviewed by H. C. Yi et al, in Journal Materials Science, 25, 1159-1168 (1990). It is concluded that almost all the known ceramic materials can be produced using the SHS method, in product forms including abrasives, cutting tools, polishing powders; elements for resistance heating furnaces; high temperature lubricants; neutron attenuators; shape-memory alloys; high temperature structural alloys; steel melting additives; and electrodes for electrolysis of corrosive media. It is acknowledged that considerable research is needed, and major disadvantages arise in "achieving high product density and tight control over the reaction and products."
This article reports numerous materials produced by SHS and combustion temperatures for some of them, viz., borides, carbides, carbonitrides, nitrides, silicides, hydrides, intermetallics, chalcogenides, cemented carbides, and composites.
Combustion wave propagation rate and combustion temperature are stated to be dependent on stoichiometry of the reactants, pre-heating temperature, particle size, and amount of diluent.
J. W. McCauley et al, in "Simultaneous Preparation and Self-Centering Of Materials In The System Ti-B-C", Ceramic Engineering and Science Proceedings, 3, 538-554 (1982), describe SHS techniques using pressed powder mixtures of titanium and boron; titanium, boron and titanium boride; and titanium and boron carbide. Stoichiometric mixtures of titanium and boron were reported to react almost explosively (when initiated by a sparking apparatus) to produce porous, exfoliated structures. Reaction temperatures were higher than 2200.degree. C. Mixtures of titanium, boron and titanium boride reacted in a much more controlled manner, with the products also being very porous. Reactions of titanium with boron carbide produce material with much less porosity. Particle size distribution of the titanium powder was found to have an important effect, as was the composition of the mixtures. Titanium particle sizes ranging from about 1 to about 200 microns were used.
R. W. Rice et al, in "Effect Of Self-Propagating Synthesis Reactant Compact Character On Ignition, Propagation and Resultant Microstructure", Ceramic Engineering and Science Proceedings, 7, 737-749 (1986), describe SHS studies of reactions using titanium powders to produce TiC, TiB.sub.2 or TiC+TiB.sub.2. Reactant powder compact density was found to be a major factor in the rate of reaction propagation, with the maximum rate being at about 60.+-.10% theoretical density. Reactant particle size and shape were also reported to affect results, with titanium particles of 200 microns, titanium flakes, foil or wire either failing to ignite or exhibiting slower propagation rates. Particle size distribution of powder materials (Al, BC, Ti) ranged from 1 to 220 microns.
U.S. Pat. No. 4,909,842, issued Mar. 20, 1990 to S. D. Dunmead et al, discloses production of dense, finely grained composite materials comprising ceramic and metallic phases by SHS combined with mechanical pressure applied during or immediately after the SHS reaction. The ceramic phase or phases may be carbides or borides of titanium, zirconium, hafnium, tantalum or niobium, silicon carbide, or boron carbide. Intermetallic phases may be aluminides of nickel, titanium or copper, titanium nickelides, titanium ferrides, or cobalt titanides. Metallic phases may include aluminum, copper, nickel, iron or cobalt. The final product is stated to have a density of at least about 95% of the theoretical density only when pressure is applied during firing, and comprises generally spherically ceramic grains not greater than about 5 microns in diameter in an intermetallic and/or metallic matrix.
U.S. Pat. No. 4,948,767, issued Aug. 14, 1990 to D. Darracq et al, discloses a ceramic/metal composite material, which may be used as an electrode in a molten salt electrolysis cell for producing aluminum, having at least one ceramic phase and at least one metallic phase, wherein mixed oxides of cerium and at least one of aluminum, nickel, iron and copper are in the form of a skeleton of interconnected ceramic oxide grains, the skeleton being interwoven with a continuous metallic network of an alloy or intermetallic compound of cerium with at least one of aluminum, nickel, iron and copper. The ceramic phase may include "dopants" for increasing its electrical conductivity and/or density. The dopants may comprise pentavalent elements such as tantalum and niobium, or rare earth metals. Inert reinforcing fibers or tissues may also be present. The method of production involves reactive sintering, reactive hot-pressing or reactive plasma spraying a precursor mixture containing a cerium oxide, fluoride and/or boride and/or at least one of aluminum, nickel, iron and copper. When used as an anode, the material is coated with a protective layer of cerium oxyfluoride. A significant disadvantage of the process disclosed in this patent arises when the constituents have widely different melting points, which makes sintering or hot pressing into a dimensionally stable product impossible. Plasma spray is a very limited technique which is unsuitable to form a large anode or similar product within a reasonable time. It is also recognized that sintering of oxide and non-oxide materials is rarely possible, and the interface bonding of materials by this technique may be inadequate for acceptable mechanical and electrical properties.
Despite the recognition of the disadvantages of prior art electrodes and the suggestion of the possibility of producing electrodes by CS, to the best of applicant's knowledge there has been no successful application of CS techniques in the production of net shaped composite electrodes for electrochemical processing which possess improved oxidation resistance while retaining adequate electrical conductivity at temperatures above 1000.degree. C.
The Yi et al article acknowledged above does not recognize or suggest the possibility of making composite electrodes by CS wherein desired properties are achieved by uniform dispersion of filler material in a ceramic or metal-ceramic core body.
It is apparent that a need exists for an electrode suitable for electrochemical processing, which exhibits improved oxidation and corrosion resistance and retains satisfactory electrical conductivity at temperatures above 1000.degree. C., and which avoids the disadvantages inherent in conventional electrodes.